Posters

We investigate the topological characteristics of a recently discovered class of semimetals in two dimensions on the honeycomb lattice. These semimetals reside at the transition between two distinct topological insulators, each existing in a nontrivial topological phase. As a result, these semimetals exhibit specific topological properties, including the presence of edge modes. In a preceding work, we demonstrated the topological robustness of this semimetal phase against disorder and interactions. In this work, we delve deeper into the semimetal's electronic properties, providing a precise calculation of its Hall conductivity and response to circularly polarized light, elaborating further on its bulk-edge correspondance leading to a half topological semimetal.

Simulating dynamics in interacting quantum many-body systems is a challenging problem. We develop a truncated Hilbert space approach (THSA) and apply it to the quantum Ising chain with both transverse and longitudinal fields, for calculating the dynamic structure factor, as well as non-equilibrium dynamics following global quenches. We find that the truncated Hilbert space approach is well suited to capture characteristic features of the model, such as E₈ particles with universal mass ratios and confinement of domain-wall excitations in the ferromagnetic phase.

Quasi-particle-interference measurements confirm the topological nature through the presence of fermi-arcs. Spectroscopic investigations reveal sample dependent electronic structure near the fermi-level, ranging from metallic characteristics to the presence of particle-hole symmetric energy gaps implying superconductivity. Most notably, the largest observed energy gap suggests a critical temperature Tc in the range of 120 K, two orders of magnitude above the previously reported Tc measured via bulk-sensitive methods. This would make trigonal PtBi₂ a potential candidate for an intrinsic topological superconductor with a Tc above liquid nitrogen temperatures, highlighting it as a promising material for technological applications in quantum computing.

Kitaev quantum spin liquid (KQSL) is an exotic state with many-body quantum entanglement which can host Majorana fermions and gauge fluxes. KQSL is an exactly solvable ground state of spin-1/2 model with the bond-dependent Ising interactions on a 2D honeycomb lattice. The presence of non-Kitaev interactions has been a primary obstacle, as they destabilize the QSL ground state. To overcome this hurdle, 3d cobaltates with more localized d-orbitals were proposed. In this study, we employed thermodynamic methods to characterize such a cobaltate KQSL candidate, Na₃Co₂SbO₆. Our investigations, includes magnetic, specific heat and thermal expansion studies on high quality single crystals. From the M(H), M(T), Cp(T) and α(T) data we mapped out the detailed anisotropic magnetic phase diagram of Na₃Co₂SbO₆. Furthermore, we address whether this compound can be driven into a KQSL ground state under an applied magnetic field or uniaxial pressure applied along the c*-axis of a single crystal.

Following the famous Afflect-Kennedy-Lieb-Tasaki model (AKLT) [1], Klein model [2], and the analysis of ground state order of extended AKLT Hamiltonians on 3D lattices [3], we construct an AKLT-like model/Hamiltonian — spin-projector Hamiltonian — on pyrochlore lattice. We consider a local spin value of S = 1 on each lattice site, which is less than the minimum value S = 3 (half of the coordination number of the lattice) required for the case of AKLT valence bond ground state (VBS). The Hamiltonian we set up is the sum of projectors onto spin 3 for every trianglular face of the tetrahedra forming the lattice. In such a model, we identify the degenerate manifold of ground states and compute an orthogonal basis in this degenerate space. Finally, we study the equal-time two-point spin correlation function to ascertain the nature of order/disorder in the ground state space.

Bi₁₂Rh₃Ag₆I₉ is the newest member of the Bi₁₄Rh₃I₉-based compounds. It can be seen as nano-periodic multilayer heterostructures of 2D topological insulators (TIs) spaced by topologically trivial insulators. The compound presents silver iodide spacers with 2D cationic conductivity. The gap of 286 meV, determined by ARPES, is the widest observed so far in a weak 3D TI. The experimental electronic structure shows good agreement with bulk band-structure calculations. The calculated invariants indicate the topological nature of the bandgap. Magnetoresistance measurements in macroscopic crystals reveal Landau quantization at low magnetic fields, coinciding with a drastic drop in resistivity across the layers. Additionally, a non-mirror symmetric dependence of the resistance on the field direction is observed at low temperatures. These effects resemble theoretical models predicting the formation of perpendicular, low-dimensional protected states emerging from the coupling of 2D edge states.

It has recently been shown that signatures of non-Hermitian topology can be realized in a conventional quantum Hall device connected to multiple current sources.
Chiral edge states are believed to be responsible for this non-Hermitian response, similar to how they lead to a quantized Hall conductivity in the presence of a single current source.
Here, we challenge this assumption, showing that multi-terminal conductance matrices can exhibit non-Hermitian topological phase transitions that cannot be traced back to the presence and directionality of a boundary-localized chiral mode.
By performing quantum transport simulations in the quantum Hall regime monolayer graphene, we find that when the chemical potential is swept across the zeroth Landau level, unavoidable device imperfections cause the appearance of an additional non-Hermitian phase of the conductance matrix.

We study the electronic structure and magnetic properties of ferromagnetic topological insulators  (Cr_xSb_1-x)₂Te₃, which exhibit Curie temperature Tc ~98 K and ~192 K for x~0.15 and ~0.35, respectively. Angle-resolved photoemission spectroscopy (ARPES) reveals a magnetic gap at the Dirac point lying ~5kBTC above the Fermi level for x~0.15. The temperature-dependent x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) showed systematic change across Tc. In addition, we observed the dichroic spin shift derived from the Cr 3d exchange split states, which are proportional to the bulk magnetization and can be simulated by charge transfer multiple cluster model calculations with an exchange field. These results provide a direction of relations of Tc and magnetization with the exchange field.

A prototypical example of a Higgs phase is a superconductor: the condensation of electrically charged Cooper pairs generates a mass for photons, resulting in the Meissener effect. However, this "condensation" is ill-defined, because the U(1) symmetry which conserves charge is pure gauge and cannot be spontaneously broken: since charges can only be created in neutral pairs, a closed system is always globally charge-neutral. To make the symmetry physical, one must have a boundary through which flux can escape, in which case the total charge in the bulk is equal to the total flux through the boundary. However, this implies that the charge conservation symmetry is actually realized on the boundary. In this work we investigate the spontaneous breaking of this boundary symmetry in the Higgs regime. I will discuss this boundary criticality in both Abelian and non-Abelian models, and comment on how it relates to recent work relating Higgs and Symmetry Protected Topological (SPT) phases.

Spiral spin liquids (SSLs) are correlated states of matter in which a frustrated magnet evades order by fluctuating between a set of (nearly) degenerate spin spirals. Here, we investigate the response of SSLs to quenched disorder in a two-dimensional Heisenberg model. At the single-impurity level, we identify different order-by-quenched-disorder (ObQD) mechanisms and analyze the ensuing spin textures. We show that, besides being extremely long-ranged, the latter generally display oscillations which can be used to reconstruct the classical ground-state manifold. At finite defect concentrations, we perform extensive numerical simulations and characterize the resulting phases at zero temperature. As a result, we find that the competition between incompatible ObQD mechanisms can lead to a spiral spin glass already at low to moderate disorder. Finally, we discuss extensions of our conclusions to nonzero temperatures and higher-dimensional systems, as well as their experimental implications.

We characterize quantum dynamics in triangular billiards in terms of five properties: (1) the level spacing ratio (LSR), (2) spectral complexity (SC), (3) Lanczos coefficient variance, (4) energy eigenstate localisation in the Krylov basis, and (5) dynamical growth of spread complexity.  The billiards we study are classified as integrable, pseudointegrable or non-integrable, depending on their internal angles which determine properties of classical trajectories and associated quantum spectral statistics.  A consistent picture emerges when transitioning from integrable to non-integrable triangles: (1) LSRs increase; (2) spectral complexity growth slows down; (3) Lanczos coefficient variances decrease; (4) energy eigenstates delocalize in the Krylov basis; and  (5) spread complexity increases, displaying a peak prior to a plateau instead of recurrences.  Pseudo-integrable triangles deviate by a small amount in these charactertistics from non-integrable ones, which in turn approximate models from the Gaussian Orthogonal Ensemble (GOE).  Isosceles pseudointegrable and non-integrable triangles have independent sectors that are symmetric and antisymmetric under a reflection symmetry.  These sectors separately reproduce characteristics of the GOE, even though the combined system approximates characteristics expected from integrable theories with Poisson distributed spectra.

We successfully established Quantum Point Contact geometries in the framework of the Topological Insulator HgTe. [1] We propose, that when gated in the regime of edge state conductance, the QPC can form an interferometer, given that the width of the constriction is small enough to allow for the last bulk mode close to the interface of the gated region to overlap with the two edge channels. In this case, several effects contributing to the quantum phase can arise. We are able to identify the impact of the Aharonov-Bohm phase as well as the dynamical Aharonov-Casher phase in the quantum spin Hall regime. Additionally, a phase shift within the first Aharonov-Bohm period gives evidence for the presence of a spin-orbit Berry phase of π, which is consistent with simple model considerations.

1) Strunz et al., Nature Physics, 16, 2019

As a new type of fundamental magnetic order next to ferromagnetism and antiferromagnetism, altermagnetism has recently attracted great attention. It is characterized by antiferromagnetic spin alignment combined with rotational lattice symmetry, which is reflected in a spin-split band structure with spin polarized electronic states. One of the "workhorse" materials potentially exhibiting this type of magnetic order is MnTe in its hexagonal NiAs-type crystal structure. Here, we investigate MnTe thin films grown on different substrates by molecular beam epitaxy. Using structural characterization methods, we discuss the influence of the growth parameters on the observed films. The electronic structure is assessed by soft X-ray angle-resolved photoemission spectroscopy and shows good agreement with band structure calculations.

[1] L. Šmejkal et al., Phys. Rev. X 12, 031042 (2022).

The successful substitution of Vanadium by more correlated Chromium atoms in the 135 compounds is widely regarded as a crucial step towards unequivocal electronically dominated physics on the kagome lattice. This hope is further fueled by the similarity of pressure dependent phase diagram with the hole doped part of the cuprate phase diagram. In our work, we show within functional renormalization group calculations, that the density wave phase around ambient pressure is driven by antiferromagnetic fluctuations, that rapidly decrease at the insulator-to-superconductor phase transition and can not account for the observed transition temperature. The superconductivity at high pressure can be recovered by considering a conventional pairing mechanism mediated by electron-phonon coupling, that is, while irrelevant in the normal state, less susceptible to the applied pressure and hence eventually overcomes electronic screening processes.

The negatively-charged boron vacancy is the first intrinsic spin defect in hexagonal boron nitride (hBN) reported by our group in 2020. Practical applications of hBN spin centers as intrinsic sensors in van der Waals heterostructures are envisioned. To further boost the quantum sensing applications of this spin defect in hBN, we investigate the dynamics of the intermediate state, which is also called metastable state because it is likely to trap electrons for a certain time. We investigated the photoluminescence dynamics and the experiments have shown that the intermediate state delays the relaxation of electrons to the ground state with a characteristic lifetime of 24 ns. This has a clear impact on the subsequent sensing protocol and must be considered when designing the pulsed optically detected magnetic resonance (ODMR) experiments.

The non-trivial topology of the quantum spin Hall insulator indenene was recently demonstrated through bulk probes that reveal its topological band ordering [1,2]. Consequently, the bulk-boundary correspondence guarantees the presence of metallic states at the edge of this triangular indium monolayer. In this study, we employ scanning tunneling spectroscopy to test indenene for this correspondence. Our results confirm metallic edge conductivity with suppressed backscattering near the bulk band gap, validating the presence of topologically protected edge states in indenene.

[1] M. Bauernfeind, J. Erhardt, and P. Eck et al., Nat. Commun. 12, 5396 (2021) 

[2] J. Erhardt et al., Phys. Rev. Lett. 132, 196401 (2024)

The bosonic fractional Chern insulator is intricately connected to quantum spin liquids. Thus, we investigate the Read-Rezayi fractional quantum Hall states for k=2 (Moore-Read) and k=3. The non-abelian excitations of the k=3-state are Fibonacci anyons and thus provide a platform for universal quantum computing. Following the analogy of the Calogero-Sutherland model, we construct parent continuum Hamiltonians for these states. Based on the identification of fractional quantum Hall states with correlation functions of CFTs, we use the Belavin–Polyakov–Zamolodchikov equation for the corresponding primary fields to find destruction operators with the ground state in their kernels and combine them to form parent Hamiltonians.

The coupling between the electronic and magnetic degrees of freedoms can lead to exotic transport phenomena in multiferroic materials. Particularly the propagation of charge neutral heat carriers can reveal interesting features in the thermal transport properties.
Here, we report the thermal conductivity measurements in multiferroic Swedenborgite CaBaM₄O₇ (M=Fe,Co) which is built up by alternating Kagome and triangular layers of edge sharing MO₄ tetrahedra in mixed valence state. We find anomalies related to the magnetic ordering in longitudinal thermal conductivity as well as non linear transversal thermal conductivity(Thermal Hall effect) resembles to that of the magnetization. We attribute the thermal hall effect of Swedenborgite CaBaM₄O₇ to phonon due to the strong spin-phonon coupling.

We investigate Kondo physics of a bath reaching from a continuum to discrete energy levels for the hybridisation function. Here, we analyze the formation of local moments in the spin and charge susceptibilities depending on the hybridisation functions. Further, we drive the system to be out of half-filling and break the particle-hole symmetry.

Symmetry-protected topological phases (SPTs) characterized by short-range entanglement include many states essential to understanding of topological condensed matter physics, and the extension to gapless SPTs provides essential understanding of their consequences. In this work, we identify a fundamental connection between gapless SPTs and recently-introduced multiplicative topological phases, demonstrating that multiplicative topological phases are an intuitive and general approach to realizing concrete models for gapless SPTs. In particular, they are naturally well-suited to realizing higher-dimensional, stable, and intrinsic gapless SPTs through combination of canonical topological insulator and semimetal models with critical gapless models in symmetry-protected tensor product constructions, opening avenues to far broader and deeper investigation of topology via short-range entanglement.

The magnetic pyrochlore lattice, a three-dimensional frustrated lattice, hosts many-body phenomena like classical and quantum order-by-disorder and spin liquids described by emergent field gauge theories. Within the most general bilinear nearest-neighbour Hamiltonian, these phenomena arise from competition between different q=0 phases, classified by their irreducible representation, . This study examines a point in the nearest-neighbour bilinear Hamiltonian on the pyrochlore lattice, parameterized by the local interaction parameter J_{z\pm}, where three symmetry-breaking phases converge. We demonstrate that for all values of J_{z\pm}, order-by-disorder phenomena induce a symmetry-breaking transition, and identified a novel spin-nematic phase emerges for a range of J_{z\pm} values. This phase is characterized using classical Monte-Carlo simulations and a self-consistent Gaussian approximation, revealing its stability in certain directions away from the point.

We present angle-resolved photoemission spectroscopy (ARPES) measurements on tunable 1-D moiré phases of an epitaxial honeycomb monolayer AgTe/Ag(111) [1]. In this model system, the moiré structure can be tuned almost continuously in contrast to hardly controllable twist angles in bilayer van-der-Waals heterostructures [2]. We experimentally observe moiré minibands and band gaps in the order of about 100 meV suggesting sizable moiré potentials. Moreover, the presented data indicates the investigated material to be a tunable model system 
which exhibits rich quantum states with long range coherence..

[1] Ünzelmann, M., Bentmann, H. et al. Orbital-Driven Rashba Effect in a Binary Honeycomb Monolayer AgTe. PRL. 124, 176401 (2020).
[2] Lisi, S., Lu, X., Benschop, T. et al. Observation of flat bands in twisted bilayer graphene. Nat. Phys. 17, 189–193 (2021).

When translational symmetry is broken, e.g. in the presence of impurities in a crystal, an ordered state can emerge where the electronic charge density modulates spatially. Using scanning tunneling microscopy (STM), we identify such charge order pinned by surface impurities. By operating the STM tip as a local gate, we demonstrate the ability to tune the spatial confinement of this charge modulation, which eventually breaks down when enough carriers are introduced to screen the charged impurities. Our work opens an avenue toward atomic-scale control of strongly correlated electrons.

Bismuthene, a honeycomb monolayer of bismuth atoms synthesized on a SiC(0001) substrate, is a topological insulator with a breakthrough bulk band gap of 800 meV due to giant spin-orbit coupling. The magnitude of this gap exposes bismuthene as a promising candidate for room temperature spintronic applications based on the quantum spin Hall effect. However, oxidation of bismuthene in air confines most experiments on this system to UHV conditions. Here we demonstrate the intercalation of bismuthene between SiC and a single sheet of graphene. This protective layer effectively prevents bismuthene from oxidation, while it fully conserves its structural and topological properties as we readily demonstrate by scanning tunneling microscopy and photoemission spectroscopy. This paves the way for ex-situ experiments and ultimately brings bismuthene closer to the fabrication of spintronic devices.

The Kagome network is a key structural motif in modern solid-state physics as it hosts special electronic states related to Dirac-cones, van-Hove singularities, and flat bands. The Vanadium based Kagome metal ScV₆Sn₆ exhibits an unconventional charge density wave below 96 K. To provide a detailed microscopic picture of the structural and electronic properties of ScV₆Sn₆ we employ 51V nuclear magnetic resonance (NMR) to investigate its CDW phase from a local point of view, aided by density functional theory (DFT). We can relate the dynamics of the local magnetic field to the changes in the V density of states (DOS), we find the reported reconstruction of the unit cell as a characteristic modulation of the charge symmetry at V nuclei, while the local magnetic field reveals a substructure of Fermi level states suggesting orbital selective modulation of the local DOS [1].

[1] R. Guehne, J. Noky, C. Yi, C. Shekhar, M. G. Vergniory, M. Baenitz, M., C. Felser, arXiv:2404.18597 (2024).

We investigate the surface properties of a layered weak topological insulator Bi₁₂Rh₃Ag₆I₉ using scanning tunnel microscopy as well as x-ray and angle-resolved photoemission spectroscopy. The compound consists of layers of two-dimensional insulators built from edge-sharing Rh-centered Bi cubes which are arranged in a Kagome-type network. Interestingly, the electronic structure shows signatures of a Kagome band structure. This is evidenced by the presence of a Dirac cone at the K-points, which is gapped by the strong atomic spin-orbit coupling, as demonstrated by ARPES measurements and DFT band structure calculations. In particular, we show that the surface band structure exhibits flat band surface states in the topological bulk band gap in close vicinity of the Fermi level.

The synthesis of itinerant Kagome materials, i.e. compounds with a metallic parent state, has initiated extensive research interests. Experimental observations indicate a fascinating phenomenology of correlated electron physics on the Kagome lattice. Deriving from sophisticated many body calculations we present a novel state of matter reminiscent of a pair density wave, i.e. a sublattice modulated superconducting pairing state emerging without additional translation symmetry breaking on the Kagome Hubbard model. Recent experiments in FeSn and KV₃Sb₅ indicate a universality of sublattice modulated superconductivity.

We study the J₁-J₂ spin-1/2 chain using a path integral constructed over matrix product states (MPS). By virtue of its non-trivial entanglement structure, the MPS ansatz captures the key phases of the model even at a semi-classical, saddle-point level, and, as a variational state, is in good agreement with the field theory obtained by abelian bosonisation. Going beyond the semi-classical level, we show that the MPS ansatz facilitates a physically-motivated derivation of the field theory of the critical phase: by carefully taking the continuum limit -- a generalisation of the Haldane map -- we recover from the MPS path integral a field theory with the correct topological term and emergent SO₄ symmetry, constructively linking the microscopic states and topological field-theoretic structures. Moreover, the dimerisation transition is particularly clear in the MPS formulation -- an explicit dimerisation potential becomes relevant, gapping out the magnetic fluctuations.

We study the bulk photovoltaic effect (BPVE) in Dirac and Weyl semimetals under two-frequency light irradiation. We show that the BPVE emerges for centrosymmetric Dirac and Weyl semimetals in the presence of light fields with frequencies Ω and 2Ω. The BPVE under the two frequency drive involves both shift current contribution independent of the carriers lifetime τ and the injection current contribution proportional to τ. Our calculations indicate that the photocurrent's direction, magnitude and type can be dynamically controlled by tuning parameters of the driving fields. Furthermore, we find that the tilt of the Dirac cone significantly affects the photocurrent, particularly in mirror symmetry-lacking Weyl semimetals, leading to an anisotropic optical response. These findings provide new insights into the dynamic control of photocurrents in topological semimetals, offering promising applications for optoelectronic devices.

Multiphase unconventional superconductivity is a rare phenomenon, which has recently been discovered in the tetragonal but locally noncentrosymmetric heavy-fermion compound CeRh₂As₂. Here, the unusual transition to a high-field superconducting phase is induced by an external magnetic field, including a non-trivial angular dependence and large critical fields. Apart from superconductivity, also other ordered phases of magnetic or electronic multipolar order have been found. Intriguingly, the transition to such a state has been reported to happen either below or above the superconducting critical temperature, with an unresolved connection to the different experimental probes. However, the probably weakly magnetic state seems to consistently coexist only with the low-field superconducting phase. In order to study the coexistence and complex interplay of the potential superconducting and magnetic phases in CeRh₂As₂, as well as the effect of applying a magnetic field, we develop a theory based on group-theoretical considerations, in combination with Bogoliubov-de Gennes and Ginzburg-Landau methods. Thereby we can give a statement about the probable symmetries of the superconducting states and their close relationship to magnetism in this material.

The Kitaev spin liquid realizes an emergent static Z₂ gauge field with vison excitations coupled to Majorana fermions. Partially motivated by layered structure of Kitaev materials, we consider Kitaev models stacked on top of each other, weakly coupled by Heisenberg interaction. This inter-layer coupling breaks the integrability of the model and makes the gauge fields dynamic. We identify novel conservation laws that keep single visons immobile. However, inter-layer vison pairs can hop within the layer, but their motion is strongly influenced by the type of stacking. For AA stacking, an interlayer pair has a 2D motion but for AB or ABC stacking, novel sheet conservation laws lead to a 1D motion. For all stackings, an intra-layer vison-pair is constrained to move out-of-plane only. Depending on the anisotropy of the Kitaev couplings, the intra-layer vison pairs can display either coherent tunnelling or purely incoherent hopping.
Ref:APJ and Achim Rosch, arXiv:2403.14284

In recent years the search for a holographic duality, which is based on tessellations of the hyperbolic plane has gained momentum and the construction of suitable boundary theories has been considered in the literature. We propose a discrete analog of JT gravity, defined on hyperbolic lattices as a dual bulk theory. We calculate the gravity path integral on the disk and show that this gives rise to an Ising model subject to a topological constraint. This constraint restricts spin domains to have a disk topology, which implies that there is only one domain possible, consisting of spin up. The resolvent of JT gravity is related to the free energy of this Ising model. We study this free energy as a function of the coupling constant through a Monte Carlo approach, and also in the mean-field approximation. Furthermore, we propose a realisation of discrete JT gravity as a matrix integral.

Altermagnets are a novel class of magnetic materials besides ferro- and antiferromagnets, where the interplay of lattice and spin symmetries produces a magnetic order that is staggered both in coordinate as well as momentum space. The metallic rutile oxide RuO₂, long believed to be a textbook Pauli paramagnet, recently emerged as a workhorse altermagnet when resonant X-ray and neutron scattering studies reported nonzero magnetic moments and long-range collinear order. While experiments on thin films seem consistent with altermagnetic behavior, the origin and size of magnetic moments in RuO₂ still remain controversial. Here we show that RuO₂ is nonmagnetic, regardless of the sample dimensions. Employing muon spin spectroscopy as a highly sensitive probe of local magnetic moments we find moments of at most 7.5×10⁻⁴ μB/Ru. Our own neutron diffraction measurements on RuO₂ single crystals identify multiple scattering as a likely source for this discrepancy.

The presence of a Dyson singularity in the density of states of one-dimensional random chiral structures is a long standing theoretical prediction. It is associated with topological marginality, and rapidly disappears in structures which move away from the phase boundary. In two-dimensional chiral systems, such as graphene, the corresponding feature is known as a Gade singularity.

Coaxial cable networks provide an excellent platform for investigating such chiral phenomena. We present here an experimental study of the Dyson and Gade singularities in small lattices. In a finite structure, the singularity is reduced to a broadened peak, which we observe by averaging over spectra from an ensemble of disordered networks, constructed from cables with different impedances. We show that, in small structures such as ours, the visibility of the feature is sensitive to the choice of boundary conditions.

Chiral superconductors (SCs) have received much attention in recent years as a promising platform for hosting Majorana-bound states due to the topological nature of their ground state.This makes them candidates for performing fault-tolerant quantum computations.
The best known candidate for chiral superconductivity is Sr₂RuO₄(SRO) [1]. Recently, this has been questioned and intensive research is still underway.
4Hb-TaS₂, a polymorph of the well-known dichalcogenide TaS₂, was identified as a candidate for chiral superconductivity [2]. We report results of muon spin relaxation studies and ultrassound spectroscopy to examine the superconducting order parameter symmetry in 4Hb-TaS₂ and study the interplay of superconductivity with other electronic degrees of freedom, such as CDW order and quantum spin fluctuations.
[1] V. Grinenko et al., Nat. Phys. 17, 748 (2021).
[2] A Ribak et al., Science Advances 6, eaax9480 (2020).

The exactly solvable Kitaev model with its frustrated bond-dependent interactions has attracted enormous attention. However, there is no pristine realization of the Kitaev model due to the significant Heisenberg and off-diagonal exchange interactions. Here we demonstrate the coexistence of the Kitaev interaction with the piezomagnetoelectric effect (simultaneous magnetoelastic and magnetoelectric responses), which can offer electric field driven manipulation of the ground state and the fractional spin excitations. Our study reports the direct observation of the magnetoelectric (ME) effect in Na₂Co₂TeO₆, and highlights the magnetoelastic response as a sensitive gauge of phase transitions. We discuss that the ME effect originates from the pd-hybridization mechanism, which allows local polarization independently from any magnetic order.

Antiferromagnetic Ising models on frustrated lattices can realize classical spin liquids, with highly degenerate ground states and possibly fractionalized excitations and emergent gauge fields. Motivated by the recent interest in curved space and AdS/CFT correspondance we study Ising models defined in negatively curved space. Specifically, we consider nearest-neighbor Ising models on tesselations with odd-length loops of two-dimensional hyperbolic space. For finite systems with open boundary conditions we determine the ground-state degeneracy, and we perform finite-temperature Monte-Carlo simulations to obtain thermodynamic data as well as correlation functions. The data collectively show that classical spin liquids that are highly dependent on the boundary conditions can be realized in hyperbolic space.

In extended Heisenberg-Kitaev-Gamma-type spin models, hidden-SU(2)-symmetric points are isolated points in parameter space that can be mapped to pure Heisenberg models via nontrivial duality transformations. Such points generically feature quantum degeneracy between conventional single-q and exotic multi-q states. We argue that recent single-crystal inelastic neutron scattering data place the honeycomb magnet Na₂Co₂TeO₆ in proximity to such a hidden-SU(2)-symmetric point. The lowtemperature order is identified as a triple-q state arising from the N ́ eel antiferromagnet with staggered magnetization in the out-of-plane direction via a 4-sublattice duality transformation. This state naturally explains various distinctive features of the magnetic excitation spectrum, including its surprisingly high symmetry and the dispersive low-energy and flat high-energy bands.

Metal-organic frameworks (MOFs) are promising candidates for advanced photocatalytically active materials. These porous crystalline compounds have large active surface areas and structural tunability and are thus highly competitive with oxides, the well-established material class for photocatalysis. However, due to their complex organic and coordination chemistry composition, photophysical mechanisms involved in the photocatalytic processes in MOFs are still not well understood. Employing electron paramagnetic resonance (EPR) spectroscopy and time-resolved photoluminescence spectroscopy (trPL), we investigate the fundamental processes of electron and hole generation, as well as capture events that lead to the formation of various radical species in UiO-66, an archetypical MOF photocatalyst. We detected a manifold of photoinduced electron spin centers, which we subsequently analyzed and identified with the help of density-functional theory (DFT) calculations. Under UV illumination, we reveal the symmetry, g-tensors and lifetimes of three distinct contributions: a surface O₂-radical, a light-induced electron-hole pair, and a triplet exciton. Notably, the latter was found to emit delayed fluorescence or phosphorescence. Our findings provide new insights into the photoinduced charge transfer processes, which are the basis of photocatalytic activity in UiO-66. This sets the stage for further studies on photogenerated spin centers in this and similar MOF materials.

In topological semimetals, Hall measurements provide an important charge transport footprint of the non-trivial geometric properties of the electronic wavefunctions. In Weyl semimetals, the planar Hall effect (PHE) -- the appearance of a transverse voltage when coplanar electric and magnetic fields are applied -- is a direct consequence of the longitudinal linear magnetoconductance associated with the chiral anomaly of Weyl fermions, and is quantified by the large Berry curvature of Weyl nodes. The anomalous Hall effect is fully determined only by the location in the Brillouin zone and topological charge of the Weyl nodes. Time-reversal invariance prohibits any anomalous Hall signal in the large class of non-magnetic Weyl semimetals thereby leaving the PHE as the only Hall diagnostic tool of Weyl physics, at least in the linear regime. This complicates the identification of non-magnetic topological semimetals by charge transport experiments.

Nodal noncentrosymmetric superconductors can host zero-energy flat bands of Majorana surface states within the projection of the nodal lines onto the surface Brillouin zone. Thus, these systems can have stationary, localized Majorana wave packets on certain surfaces, which may be a promising platfrom for quantum computation. However, for such applications it is important to find ways to manipulate the wave packets in order to move them without destroying their localization or coherence. As the surface states have a nontrivial spin polarization, applying an exchange field, e.g., by introducing a magnetic insulator at the surface, makes the previously flat band slightly dispersive. We aim to use an adiabatic change of the exchange field to move wave packets on the surface. We therefore investigate the time evolution of a maximally localized wave packet under the influence of such an exchange field employing exact diagonalization as well as quasiclassical approximations.

The Ferromagnetic Kondo Lattice Model (FKLM) describes systems with interactions between localized and itinerant electrons and can host noncoplanar spin textures that give rise to phenomena such as the Quantum Anomalous Hall Effect (QAHE). In this work, we develop an optimization-based numerical program to perform an unbiased and comprehensive search for the ground state spin texture under varying conditions. We confirm the stability of noncoplanar spin textures, particularly at ¼ filling, and identify the tetrahedral spin texture as the ground state over a range of exchange couplings. Our optimization-based approach provides a powerful tool for exploring complex physical phenomena like the QAHE and skyrmions, paving the way for future research in these areas.

Topologically ordered spin liquids represent a class of phases with intriguing characteristics such as a ground-state degeneracy varying with the space's genus and anyons. Recent advancements in hyperbolic lattices, discrete analogues of negatively curved spaces, have opened new avenues for extending Euclidean lattice models to them. Given the high genus implied by negative curvature, this raises fundamental questions about the interplay between hyperbolic lattices and topological order with potential impact on quantum error correction. I will present a generalization of Kitaev's honeycomb model to hyperbolic lattices which maximally preserves symmetry, enabling us to determine the ground-state sector exactly and explore the phase diagram and vortex lattices efficiently. Unlike in the Euclidean case, the gapless spin liquid exhibits a non-zero density of states at zero energy. Finally, I will discuss properties of the chiral-spin-liquid phase induced by time-reversal-symmetry breaking.

Ca₃Ru₂O₇ is an antiferromagnetic (AFM) polar metal considered a fascinating material due to its wide range of remarkable electronic phenomena, including colossal magnetoresistance, spin waves, and multiple phase transitions. Exploring these properties under external manipulation, such as electrical current, pressure, or strain, opens new pathways for understanding its electronic behavior and controlling its quantum states. In this study, we employ ab initio methods to investigate the stability of several AFM configurations of Ca₃Ru₂O₇ under lattice deformation (pressure and strain). We identify potential altermagnetic (AM) states, a recently discovered elemental magnetic phase, and show that these states can be stabilized under strain. We discuss the underlying mechanisms responsible for the stability of the AM phase and propose a novel route for tuning quantum states through AFM to AM transitions in Ca₃Ru₂O₇ under lattice deformation.

We propose a phase-biased non-Hermitian Josephson junction (NHJJ) composed of two superconductors mediated by a short non-Hermitian link. Such a NHJJ is described by an effective non-Hermitian Hamiltonian derived based on the Lindblad formalism in the weak coupling regime. By solving the Bogoliubov-de Gennes equation, we find that its Andreev spectrum as a function of phase difference exhibits Josephson gaps, i.e. finite phase windows with no Andreev (quasi-)bound states. The complex Andreev spectrum and the presence of Josephson gaps constitute particular spectral features of the NHJJ. Moreover, we propose complex supercurrents arising from inelastic Cooper pair tunneling to characterize the anomalous transport in the NHJJ. Additional numerical simulations complement our analytical predictions. We demonstrate that the Josephson effect is strongly affected by non-Hermitian physics.

Miniaturised photonic circuits have been employed in a broad field of fundamental, but also application-oriented research in the recent years. While the harnessing of CMOS fabrication techniques has enabled precise sub-wavelength feature control, the dynamic switching of nano-optical devices often involves additional thermo-optical or electro-optical elements. The present contribution demonstrates an all-optical switch utilising two counter-propagating beams to control the in-coupling efficiency of a nano-optical grating. The findings of the fundamental experiment are shown to support the development of a range of applications including all-optical logic gates and non-Hermitian photonic circuits. The demonstrated coupling control enables complexity reduction in integrated photonic circuits and provides a promising approach for a new, streamlined device architecture.

A characteristic feature of the superconducting state in BCS theory is the appearance of a full gap in the quasiparticle spectrum. Under various conditions, one can instead obtain an exotic form of superconductivity for which the superconducting gap contains Bogoliubov Fermi surfaces (BFSs). A BFS is a 𝑑 − 1-dimensional surface of zero-energy states in the 𝑑-dimensional momentum space. BFSs were recently observed in the two-dimensional heterostructure Al-InAs in an applied in-plane magnetic field [1]. We present the theoretical prediction for the density of states of such a system and predict the temperature dependence of observables such as the heat capacity and the superfluid density in the presence of BFSs.
[1] Phan et.al, Phys. Rev. Lett. 128, 10770 (2022).

We develop a formalism to study the effect of strong electron-electron interactions in a Weyl semimetal. In this poster, we present the findings for the case of two doped Weyl cones with opposite chirality. For this purpose, we employ a path integral formalism to study different instabilities that could take place.

Although the transitions from topological states to symmetry breaking states with trivial topology have been discussed, the road from one topological ordered state to another with the same Hall conductance and broken translational symmetry has not been found. Here we show the intriguing evidence that the FQAH to FQAH Smectic (FQAHS) transition is robustly realizable in the archetypal correlated flat Chern-band model at filling ν = 2/3. This transition is novel in that: i) the FQAHS acquires the same fractional Hall conductance as FQAH, which cannot be explained by mean-field band folding. The formation of smectic order can be viewed as perturbation around the transition point, and thus, do not destroy or change the original topology; ii) the charge excitation remains gapped across the transition although the neutral gap is closed at transition point; and iii) the transition is triggered by the softening of roton mode with the same wave vector as the smectic order.

The generation and manipulation of topological states in 1-3D photonic systems show intriguing properties and unique applications. For the 1D system, we explore the selective excitation of topological trivial/nontrivial states by tuning the symmetry and/or dynamic phase of the topological zero mode. For the 2D system, ultralow-threshold polariton condensation at BICs in organic semiconductor photonic crystals is investigated. For the 3D systems, the berry phase acquired in the Möbius ring and microtubular resonators are studied. In addition, non-Hermitian photonics is studied by introducing perovskite materials as the gain medium. These studies are of high interest for both fundamental research and practical applications such as quantum computing and optical communications.

Motivated by the recent reports on quantum spin liquids in spin-1 three-dimensional systems, we re-address the projective symmetry group framework existing for spin-1/2 in a more generic sense based on a fermionic representation of spin-S. We discuss the difference between integer and odd-half integer spin systems. Then we implement the formalism for spin-1 diamond lattice and tabulated all possible Ansatze for fully symmetric quantum spin liquid. Furthermore we employ J₁-J₂ Heisenberg antiferromagnetic model and produce the phase diagram. Interestingly, the phase diagrams completely depends on the the constraints which are required to restore the physical Hilbert space in the fermionic picture in contrarary to the spin-1/2 system. Our study sets a stage for further investigation from variational monte carlo point view for more detailed understanding on such strange behaviour.

Entangled photons are crucial for quantum technologies, but their transport through optical devices is often hindered by defects or perturbations that degrade the entanglement. Photonic floquet waveguides offer a promising solution by exploiting the robustness of topologically protected transport to ensure quantum information transport. Theoretical model predicts robust quantum information transport in such systems, but practical evidence for robust transport of entangled photons is still lacking due to the challenges of injection, detection, and measurement within a topological framework.
This project aims to study the robust transport of entangled photons, specifically the N00N state, through the Floquet system. This system consists of an array of evanescently coupled helical waveguides arranged in a graphene-like honeycomb lattice that exhibits topological behaviour. In my talk, I will discuss our progress, present findings, and the challenges encountered along the journey.

Using the example of 3D Weyl semimetals, we show that the nodal tilt in relativistic semimetals can be used as a resource for designing electronic lensing devices. Intriguingly, the lensing can formally be mapped to the gravitational lensing associated with cosmological black holes. We therefore also discuss the implications and limitations of this mapping between solids and black holes.

Topological Insulators (TIs) exhibiting the Quantum Spin Hall effect (QSHE) have been recognized as promising candidates for next-generation electronic devices due to their dissipationless and spin-polarized electronic properties. To be suitable for device applications, TIs must exhibit specific characteristics such as scalability, reproducibility, tunability, and robustness of the helical edge channels beyond liquid Helium temperatures. Although many materials have been predicted to host the QSHE, only a few have demonstrated the underlying transport signatures, but no material platform checked all the other characteristics mentioned above. Here, we present a TI based on an InAs/GaInSb/InAs trilayer quantum well exhibiting the QSHE even at elevated temperatures up to 60K, thereby all requirements are now met for the III-V semiconductor heterostructures. Our findings pave the way for the integration of TIs based on the InAs/GaInSb material system in topological field-effect devices.

Isolated magnetic impurities can be used to probe the low-energy properties of a host system, with the standard Kondo effect in metals being the paradigmatic example. Magnetic impurities have also been discussed as probes of quantum spin liquids and their excitations, and various approximate theoretical treatments have been put forward. In particular, it has been suggested that a spin liquid with a spinon Fermi surface would lead to Kondo screening akin to that in normal metals. Here we study this problem for a particular Kitaev model with a Majorana Fermi surface. We present a numerically exact solution using Wilson's Numerical Renormalization Group (NRG) which generalizes previous work for the honeycomb-lattice Kitaev model. Our numerical data for the renormalization-group flow and for thermodynamic observables highlight important differences between the Kitaev system and a metal, related to the fractionalization scheme and the influence of the emergent gauge field.

Fe₄GeTe₂ is a van der Waals material garnering significant attention due to its high temperature of ferromagnetic transition (~270 K), attributed to the strong interaction between Fe spins. The Tc of Fe₄GeTe₂ is notably sensitive to sample thickness and Fe concentration, offering opportunities for property tuning.
This study investigates the evolution of the crystal structure of Fe₄GeTe₂ with temperature using x-ray single crystal diffraction. Although, no strong structural changes were observed across the range from 20 to 320 K, a stable superstructure with a wave vector q = (1/3, 0, 0) was discovered for the first time. Additionally, signs of magnetoelastic coupling were detected. These findings contribute to a deeper understanding of the structural and magnetic behaviors of Fe₄GeTe₂, potentially guiding the development of tunable ferromagnetic 2D materials.

Entanglement measures have proven useful in characterizing novel quantum many-body states. Particularly, the scaling of bipartite entanglement entropy (EE) of a spatial subsystem with its size carries a fingerprint of underlying phases and is sensitive to quantum phase transitions. However, analytical methods to calculate EE have been limited to non-interacting theories, or theories with conformal symmetry in 1+1D. Numerical methods are applicable to more generic interacting systems, but are limited by the exponentially growing complexity of the problem. Adapting recent Wigner-characteristic based techniques, we show that Renyi EE of interacting fermions in arbitrary dimensions can be represented as a Schwinger-Keldysh free energy on replicated manifolds with a current between the replicas. The current is local in real space and present only in the subsystem of interest. This allows us to construct a diagrammatic representation of EE in terms of connected correlators in the standard unreplicated field theory. We further decompose EE into "particle" contributions which depend on the one-particle correlator, two-particle connected correlator and so on. We apply our formalism to calculate the second REE in the ground state of a 2D repulsive fermi gas in continuum within perturbation theory in coupling strength. We find that the interacting answer scales as the free-Fermion answer in space, however, the scaling prefactor increases with interaction strength. We believe this enhancement is non-universal and we will show how one-particle corrections alone cannot account for it.

In our studies we measured magnetic domains in MnSb₂Te₄ using STXM. We found a strong history and measurement protocol dependency of their size and shape. In some protocols we were able to investigate circle shaped domains eventually supporting an interpretation as a skyrmion phase that was stable over a wide range of temperature and field variation.

A quantum system governed by a non-Hermitian Hamiltonian may exhibit zero temperature phase transitions that are driven by interactions, just as its Hermitian counterpart, raising the fundamental question how non-Hermiticity affects quantum criticality. In this context we consider a non-Hermitian system consisting of an XY model with a complex-valued four-state clock interaction that may or may not have parity-time-reversal (PT) symmetry. When the PT symmetry is broken, and time-evolution becomes non-unitary, a scaling behavior similar to the Berezinskii-Kosterlitz- Thouless phase transition ensues, but in a highly unconventional way, as the line of fixed points is absent. From the analysis of the d-dimensional RG equations, we obtain that the unconventional behavior in the PT broken regime follows from the collision of two fixed points in the d → 2 limit, leading to walking behavior or pseudocriticality.

The transition metal dichalcogenide compound TiSe₂ transforms into a 2x2x2 charge density wave (CDW) phase below 200K. Se 4p valence band and Ti 3d conduction band hybridize and form CDW gap. These hybridized orbital characters are crucial information to understand the origin of CDW property. Recent studies show that linear- and circular dichroism (LD and CD) in angle-resolved photoelectron spectroscopy (ARPES) provides insights into the orbital texture of the initial states. Here, using ARPES in the soft x-ray photon energy range, we systematically study the LD and CD of TiSe₂ both with and without CDW phases. The LD and CD of Ti 3d conduction band show similar results irrespective of CDW phase. We found that CD features of Se 4p band top resemble with those of Ti 3d, indicating the mixed orbital character of Se 4p and Ti 3d.

Recent progress in the fabrication and control of layered materials in the mono- or few-layer limit have reinvigorated the study of the fundamental properties of excitonic states. However, while the binding energy and optical activity of excitons has been studied in a lot of detail, much less is known about the topological and quantum geometric properties of the excitonic band, despite the recent realization of its central role in single particle properties and responses. This is partly due to the lack of a proper characterization of the topological properties of the exciton bundle. In particular, it is unclear how the topology of the underlying electron and hole bands are related to the resulting topology of the exciton band structure in the presence of interaction. Here, we develop a comprehensive framework that characterizes the geometric properties of excitonic states based on the full lattice wavefunctions. Specifically, we clarify how the electron-hole interaction can affect the topology of the bound state. This becomes possible in a generalized framework which entirely sidesteps the effective mass approximation, which is almost universally used in low-energy effective models to obtain analytical solutions. As our main result, we identify two gauge-invariant quantities that fully characterize the exciton’s topology and geometry: The exciton shift vector SQ associated with the center-of-mass momentum Q and the exciton dipole vector Pp, which is connected to the relative momentum p. The shift vector encodes how the electron-hole interaction influences the exciton band topology, which makes it possible to fully characterize the latter. Somewhat suprisingly, we find that the shift vector gives rise to a non-trivial polarization, which might be observable for finite exciton densities at elevated temperature. On the other hand, the dipole vector is a polarization vector associated with the exciton’s intrinsic dipole moment, which couples to an external electric field. The exciton dipole vector thus plays a crucial role in exciton dynamics, giving rise to a new anomalous velocity term in the exciton’s semiclassical equations of motion of the exciton.

Exhaustive study of topological semimetal phases of matter in equilibriated electronic systems and myriad extensions has built upon the foundations laid by earlier introduction and study of the Weyl semimetal, with broad applications in topologically-protected quantum computing, spintronics, and optical devices. We extend recent introduction of multiplicative topological phases to find previously overlooked topological semimetal phases of electronic systems in equilibrium, with minimal symmetry-protection. We look into the multiplicative counterpart of the Weyl semimetal and find rich and distinctive bulk-boundary correspondence and response signatures that greatly expand understanding of consequences of topology in condensed matter settings, such as limits on Fermi arc connectivity and structure and transport signatures such as the chiral anomaly.

Frustrated magnetism is highly influenced by the geometry of the lattice and the competing interactions between the magnetic atoms in a system. In this poster, we will put forward results and progress done in studying these systems. Kobyashevite, Ktenasite and Posnjakite occur as natural minerals which were synthesized as powders via hydrothermal method. X-ray diffraction done at room temperature describe these materials to be formed of magnetic and non-magnetic layers. Within the magnetic layer, Cu-atoms are arranged in a triangular arrangement making them frustrated. Low temperature magnetic studies done upto 2K revealed that Kobyashevite and Ktenasite undergo an antifferomagnetic transition at ~ 3.14 and 4.12 K respectively. Interestingly, Posnjakite did not show any such transition within the measured range. The frustration parameter calculated is as high as 25 in these systems. Specific heat studies and neutron diffraction experiments are currently ongoing as well as planned.

Spinon-magnon mixing was recently reported in botallackite Cu₂(OH)Br with a uniaxially compressed triangular lattice of Cu²⁺ quantum spins

[1]. Its nitrate analogue rouaite, Cu₂(OH)₃(NO₃), has a highly analogous structure and might be expected to exhibit similar physics. We grew

cm-scale single crystals of rouaite hydrothermally, and achieved >90% deuteration to allow neutron scattering studies of its magnetic order.

We report rouaite’s magnetic phase diagrams for H || a, b, and [001]. The lowest-temperature magnetic state comprises alternating ferro- and

antiferromagnetic chains, similar to botallackite but with different canting, while the higher-temperature phase is an approximately helical

modulation of this. The hierarchy of exchange interactions indicates that rouaite is quasi-one-dimensional but we do not observe spinons,

possibly because the material is more two-dimensional magnetically than botallackite.

The purpose of this work is to formulate a kinetic theory for the investigation of interaction effects on transport properties of electrons in the Landau-level states. Following Keldysh formalism, we derive the quantum kinetic equation using the Landau-level basis. This equation can be use to study transport properties of interacting electrons in a constant background magnetic field of arbitrary strength. As an application of our quantum transport equation, we calculate magneto-thermoelectric coefficients of a disordered two-dimensional electron gas (2DEG) and 3d Dirac/Weyl semimetals in the quantum hall regime interacting with acoustic phonons.

The functional renormalization group (FRG) approach for spin models relying on a pseudo-fermionic description has proven to be a powerful technique in simulating ground state properties of strongly frustrated magnetic lattices. However, the FRG as well as many other theoretical models, suffer from the fact that they are formulated in the imaginary-time Matsubara formalism and thus are only able to predict static correlations directly. Nevertheless, describing the dynamical properties of magnetic systems is a theoretical challenge, as they are the key to bridging the gap to neutron scattering experiments. We remedy this shortcoming by establishing a methodical approach based on the Keldysh formalism, originally developed to handle non-equilibrium physics.
This approach allows for calculating the dynamic properties of spin systems on arbitrary lattices. We can identify the correct low-energy dynamic spin structure factors for examplary Heisenberg systems and the Kitaev Honeycomb model.

We explore the phenomenon of exchangeless braiding in Kitaev chain systems. Kitaev chains, known for their potential to host Majorana zero modes, provide fertile ground for investigating non-Abelian statistics. Our research demonstrates the manipulation of Hamiltonian parameters can facilitate braiding purely through quantum state evolution, avoiding the challenges of physically moving particles. The results offer insights into the theoretical implementations and potential applications of exchangeless braiding in topological quantum computing.

Non-Hermitian Hamiltonians allow for an effective description of dissipative systems. They exhibit a variety of exciting phenomena that cannot be observed in the Hermitian realm. Exceptional Points (EPs) are a prime example of this. At EPs not only the complex eigenvalues, but also the eigenvectors coalesce and sensitivity to perturbations is enhanced. This concept has recently found fertile ground in optics and photonics where non-Hermitian eigenstates can be induced by optical gain and loss. Similar control of x-rays is desirable due to their superior penetration power and detection efficiency. Here, we investigate theoretically non-Hermitian x-ray photonics in thin-film cavities probed by x-rays under grazing incidence. These cavities present loss that can be controlled via the incidence angle of the x-rays. Application of a magnetic hyper- fine field paves the way to tune the system towards EPs and to explore their rich topological properties.

Integration of high Q resonators for lossy materials.

1D and 2D structures of materials with topological electronic bands are beneficial for better visualizing enhanced surface transport and their utilization in quantum nanotechnology. TaAs₂, NbAs₂, and TaSb₂ are classified as weak topological semimetals and predicted to harbor rotational-symmetry-protected topological crystalline insulator (TCI) phase with surface dispersion of type-II Dirac fermions. Studying nanowires/nanoribbons (NWs/NRs) of this family of materials is desirable. Here, we present the synthesis and characterization of single-crystalline, TaAs₂ NWs/NRs with unique core-shell structures. Further, NWs/NRs are integrated into four-terminal devices, and their magnetotransport properties have been studied. We will also present the epitaxial growth 3D-Dirac semimetal Cd₃As₂ planar nanowires on different planes of the sapphire (α-Al₂O₃), resulting in well-aligned arrays suitable for integration into devices.

The recently discovered MnBi₂Te₄ family holds significant potential for applications in spin-based technologies due to the unique quantum phenomena derived from their topological properties. A crucial factor is cationic intermixing, which in the compounds affects the electronic and magnetic properties. In this study, we utilize nuclear magnetic resonance and muon spin spectroscopy, along with Density Functional Theory (DFT) calculations, to explore the impact of intermixing on the magnetic properties of MnBi₂Te₄(Bi₂Te₃)n and MnSb₂Te₄. Our findings confirm that the intrinsic and antisite Mn magnetic moments are oppositely aligned in the ground state for all family members. Additionally, we demonstrate that the static magnetic moment on the Mn antisite sublattice disappears below the ordering temperature, resulting in a homogeneous magnetic structure unaffected by intermixing. This discovery suggests a path for optimizing the essential surface-state magnetic gap.

Rare earth metals (REMs) are difficult, if not impossible, to clean as bulk materials due to their extreme reactivity. Therefore, films of REMs are studied to explore their rich magnetic properties, which are primarily influenced by the element-specific sign and wavelength of the RKKY interaction. Based on their complex cleaning procedure, the magnetic structure of REM surfaces remains a subject of ongoing debate, while it is still unknown for most of them. Here we present investigations of the structural, electronic, as well as the complex magnetic properties of Europium (Eu) films epitaxially grown on W(110), using spin-polarized scanning tunneling microscopy (SP-STM).

Europium (Eu) has a half-filled 4f shell with no d-electrons, adopting a body-centered crystal structure in bulk. However, in thin epitaxial films, a metastable hexagonal close-packed structure is expected, accompanied by helical spin ordering below its Néel temperature of 91 K. With optimal preparation conditions, we successfully grew clean, smooth films, where we observed the exchange-split surface state exclusively in the unoccupied region. Beyond a critical film thickness, striped regions with a periodicity of approximately 3 nm were identified. Experiments with differently magnetized STM tips and the application of an external magnetic field up to ±2.5 T revealed the magnetic nature of these stripes. By varying the bias voltage, we were able to distinguish between structurally and magnetically induced stripe patterns.

We present muon spin rotation (µSR) studies showing that long-range magnetic order occurs in RuBr₃ at ~ 34 K. The magnetic ordering is robust and static suggesting more conventional at zero field. Present investigations prove in RuBr₃ the Kitaev interactions are likely to be weakened at zero field compared to α-RuCl₃. The Kitaev interactions can be tuned by replacing Cl with heavier halogen elements such as Br.
NaYbS₂ is a candidate material for a pseudo-S = ½-based quantum spin liquid on the ideal triangular lattice. Single crystals of the solid solution series NaYb₁-xLuxS₂ are synthesized in the range 0 3+ by Lu₃+ ions with no sign of magnetic ordering down to 2 K suggesting a diluted triangular spin liquid. We investigate this series by µSR experiments to study the magnetism and make the time-field scaling relation to identify the presence or absence of exotic excitations.

The classification of operator algebras according to von Neumann provides insights into the entanglement structure of quantum field theories. We investigate the decomposition of subregion operator algebras of different types in the presence of a conserved charge. Drawing motivation from the concept of symmetry resolution of entanglement, intensively investigated in quantum field theories and holography, we expect a direct sum structure of operators which are invariant under the corresponding symmetry. We study the classification of these sectors along the lines of the Araki-Woods construction.

Intermetallic antimonides containing rare-earth (R) and transition (T) metals show various compositions and structures. A representative group is formed by RTSb₃ compounds. Single crystals of these can be grown by various methods. This study used a Bi flux synthesis and hot centrifugation to access a new Fe-containing RTSb₃ representative. Single crystals of RFeSb₃ with R = Pr,Nd,Sm,Gd and Tb have previously been investigated, while the isostructural Ce-member was newly discovered in this work. The RFeSb3 structure consists of double layers of FeSb₆ octahedra alternating with R and Sb square layers. These compounds display metallic conductivity and anisotropic magnetic properties, suggesting anti-ferromagnetic ordering of the R substructure without any contribution of the Fe substructure to the magnetic properties. This is supported by the Mössbauer spectra of CeFeSb₃ at room temperature and 4.2 K. Further investigation of CeFeSb₃'s magnetic and transport properties is ongoing.

The interplay of 3d and 4f magnetism in rare earth transition metal antimonides gives rise to complex magnetic properties. In R₃Fe₃Sb₇ (R = Pr, Nd) iron and rare earth atoms form columns of stacked triangles separated by antimony.
Iron exhibits non-collinear ferromagnetic order below 𝑇_c ≈ 380 K. On cooling at the onset of rare earth order a spin reorientation (𝑇_SRT, Pr ≈ 40 K,T_SRT, Nd ≈ 50 K) and magnetization reversal is observed.
Mössbauer spectroscopy is employed to resolve the local magnetic structure.
Mössbauer spectra reveal two distinct magnetic iron sites between 𝑇_c and 𝑇_SRT, both collapsing into one new site below 𝑇_SRT.
Transverse field Mössbauer measurements have been performed to investigate turning of iron moments in the ab plane at room temperature. Turning of the ab components of the local moments in 0.1 T TF field is observed.
We will discuss the implications of our findings on the magnetic structure of the system.

The realm of topological Kagome magnets offers a rich field for exploring the intricate interplay of quantum interactions among geometry, topology, spin, and correlation. The combination of magnetic interactions with nontrivial band structures can lead to the emergence of various exotic physical phenomena such as an anomalous Hall, anomalous Nernst effect. HoPtSn crystallizes in a hexagonal crystal structure with the P-62m space group with two antiferromagnetic transitions (Néel temperature ~7.4K & 3K). This structure features distorted Ho-based Kagome nets. In this work, we observe two distinct metamagnetic transitions when magnetic field, H // c-axis. A large anomalous Hall conductivity of 1300 Ω−1 cm−1 at 2 K and topological Hall effect is observed for HoPtSn crystals. Our findings suggest that RTX family could serve as an excellent platform for investigating the intricate relationship between magnetic and electronic structures, paving the way for exploring novel quantum phenomena.

ZrTe₅ is known for its large thermopower and resistivity anomaly and offers a promising platform to study topological phase transition. Recent years have witnessed non-trivial and complex electronic properties that assigned this material to either a strong topological insulator or a 3D Dirac semimetal phase. Here, I will present the electronic properties of ZrTe₅ depicting different resistivity anomaly temperature Tp in distinct crystals. Strong temperature dependence of band-structure has also been observed in p- to n-type transition across the resistivity anomaly in Hall and Seebeck coefficient measurements.

We explore the electromagnetic response of time-reversal (TR) invariant and TR breaking Weyl superconductors (SCs). It is known that Weyl semimetals contain Weyl nodes in their band structure — a property captured in the low energy effective theory by an axion term in the action. In the case of a Weyl SC this behavior persists but has to be supplemented by traditional terms describing SCs. Considering the theory Weyl SCs in the London limit we show that that TR invariant Weyl SCs feature a chiral Meissner state, where an additional component of the magnetic induction is generated inside the SC. This leads to other consequences in the electromagnetic response different from conventional SCs, namely, the existence of a critical value of the axion coupling where the screening of the magnetic field breaks down. Contrary, in the TR breaking Weyl SCs the magnetic field screening is strengthened by the axion contribution, but the latter results in an induced electric field inside the SC.

Quantum emitter arrays are a powerful platform enabling tailored control of quantum optical phenomena, like super- and subradiance or
efficient photon storage. Since state-of-the-art experimental techniques allow the realization of almost arbitrary lattice structures, a natural question is what physical effects arise if the lattice has nontrivial topology. Here, we study a one-dimensional chain of quantum emitters im-
plementing the Su-Schrieffer-Heeger model. Going beyond previous studies, we show how the presence or absence of topologically pro-
tected edge states depends on the orientation of the transition dipole moment with respect to the chain axis. Our results demonstrate the potential of atomic emitter arrays as a platform for topological quantum optics.

Altermagnets are a newly recognized type of collinear magnets that have vanishing net magnetization yet lift Kramers spin degeneracy in their band structure. We study the impact of external electric and magnetic fields on the Josephson response of a planar superconductor/altermagnet/superconductor junction. We show that the critical Josephson current across the junction oscillates as the altermagnetic strength increases. Meanwhile, the current-phase relation can be forward- or backward-skewed and can be changed significantly by tuning the altermagnetic strength. Moreover, we reveal that the effects of the external fields are twofold: (i) it can induce 0-π transition as the field strength grows, thus facilitating the application of altermagnetic Josephson junctions; and (ii) the amplitude of the critical current can be significantly enhanced at finite field strengths. These findings pave the way for exploring further applications of superconductor/altermanget heterostructures.

Recent achievements in the quantum anomalous Hall effect (QAHE) in MnBi₂Te₄ and MnBi₄Te₇ position the (MnBi₂Te₄)(Bi₂Te₃)n family as a promising platform for QAHE advancements due to their ferromagnetically (FM) ordered MnBi₂Te₄ septuple layers (SLs). However, QAHE realization is challenging because of substantial antiferromagnetic (AFM) coupling between the SLs. Stabilizing an FM state, beneficial for QAHE, can be achieved by intercalating SLs with additional Bi₂Te₃ quintuple layers (QLs), though the mechanisms and required QL number are unclear, and surface magnetism remains obscure. This study demonstrates robust FM properties in MnBi₆Te₁₀ (n = 2) with a Curie temperature (Tc) of ~12 K, elucidated through combined experimental and theoretical approaches. Our measurements reveal a magnetically intact surface with a large magnetic moment, comparable to the bulk. This establishes MnBi₆Te₁₀ as a promising candidate for QAHE at elevated temperatures.

In this work, we study the matrix element effect of the angle-resolved photoemission spectroscopy (ARPES) intensity to deduce k-space initial state orbital texture featuring different symmetries in type-II Dirac semimetal PtTe₂. We observe the switching of the orbital texture of the Dirac surface state relative to the Dirac point. Our spin- and angle-resolved photoemission data were augmented by the one-step model of the photoemission within the spin-polarized relativistic Korringa-Kohn-Rostoker (SPR-KKR) Green’s function method. To extract information about the different contributions to the resulting spectral weight and spin-polarization, the matrix element used in our one-step model of photoemission calculations includes all experimental parameters such as photon energy, light polarization and geometry configurations. Via such control over the experimental parameters, to investigate the orbital wavefunction above and below the Dirac point, we performed polarization-dependent ARPES.

We investigate ring-shaped microstructures of HgTe based quantum well heterostructures in electrical magneto transport experiments. To achieve this, advanced lithography techniques are employed to pattern the heterostructure layer stack using an isotropic wet chemical etching process. We plan to separately contact inner and outer edge channels of the ring structure using side contacts and an air bridge technology. Subsequently, low-temperature transport measurements will be conducted to investigate the quantum spin Hall edge channels in our 4-terminal devices. On our poster we will present the initial findings of this research.

Novel 2D quantum materials display very interesting topological properties, such as a Quantum Spin Hall phase up to room temperature in monolayers bismuthene and indenene. However, transport measurements and nanofabrication are often impossible due to the monolayers extreme air-sensitivity.

In order to fabricate nanostructures of air-sensitive monolayers, a unique UHV nanofabrication setup based on stencil lithography (using pre-developed hard Si stencils) is being develloped at LNCMI. This compact setup comprises a positioning stage to align sample and stencil, evaporation cells for electrical connection and capping layer deposition, and an ion source for ion-beam etching. This way, a thin film grown in UHV by collaborators can be brought through a UHV-suitcase, and nanopatterned onsite and in-situ, before being capped to enable measurements ex-situ. This setup will allow to fabricate nanostructures of any air-sensitive material to be measured in very high magnetic fields.

In this experimental study, we use scanning tunneling microscopy and spectroscopy to investigate Yu-Shiba-Rusinov states induced by 4f-shell rare-earth Gd adatoms on a superconducting Nb(110) surface. We engineer Gd atom chains along the substrate's [1-10] and [001] directions, revealing distinct behaviors in differently oriented chains. -oriented Gd chains exhibit spectroscopic features at their ends, identifying them as trivial edge states, while [001] -oriented Gd chains display zero-energy edge states, suggesting non-trivial nature. Notably, Gd chains with four atoms--independent of their particular orientation--exhibit a uniform zero-energy mode along the entire chain. These findings call for further research and a theoretical framework to describe rare-earth-based structures on superconductors.

Fixed-point annihilation is a generic mechanism that generates an extremely slow RG flow and has been suggested to explain the occurrence of a weak first-order transition instead of a deconfined critical point in two-dimensional quantum magnets. Here we explore this phenomenon in a (0+1)-dimensional spin-boson model which can be solved with unprecedented numerical accuracy using the recently-developed wormhole quantum Monte Carlo method. We find a tunable transition between two ordered phases that can be continuous or first-order, and even becomes weakly first-order in an extended regime close to the fixed-point collision. We provide direct numerical evidence for pseudocritical scaling on both sides of the collision manifesting in an extremely slow drift of critical exponents. We also find scaling behavior at the symmetry-enhanced first-order transition as described by a discontinuity fixed point. Our study motivates future work in higher-dimensional quantum dissipative spin systems.

The series of RAlSi with R being a rare earth element is of high interest as Weyl nodes are induced in those materials by broken inversion symmetry. LaAlSi is a nonmagnetic Weyl semimetal, while CeAlSi shows canted Ferromagnetism and in NdAlSi as well as in SmAlSi, spiral spin structures are found.
We used NMR experiments to gain local information about the magnetic order parameter and excitations of RAlSi (R = {La, Ce, Nd}). Surprisingly, in this isostructural series we observe an electric field gradient (EFG) at the Al position, which depends on the actual rare earth element present in the compound. The EFG is strongest for NdAlSi and vanishes for LaAlSi. This is surprising since the 4f-electrons are typically strongly localized inside the rare earth ion. In most cases the EFG can be well described using the charge distribution of the investigated atom and the total charge of surrounding atoms, which is the same for all rare earth elements.

In this project, we theoretically identify and investigate a promising copper-based kagome metal candidate, yet to be experimentally realized. We endeavor to engineer a kagome metallic phase exhibiting the smallest possible multi-orbital character, thereby bridging the gap between theoretical modeling and experimental realizations. The ideal candidate is found in the CsCu₃CI₅ compound. Our results were obtained employing both ab-initio calculations, in the framework given by density functional theory (DFT), as well as crystal field analysis. Remarkably, we generically obtain a mixed-type van Hove singularity in close proximity to the Fermi level. Our proposed material promises to exhibit exotic electronic features, opening new possibilities for exploring unprecedented quantum phenomena in kagome metals.

We introduce methods of characterizing entanglement, in which entanglement measures are enriched by the matrix representations of operators for observables. These observable operator matrix representations can enrich the partial trace over subsets of a system’s degrees of freedom, yielding reduced density matrices useful in computing various measures of entanglement, which also preserve the observable expectation value. We focus here on applying these methods to compute observable-enriched entanglement spectra, unveiling new bulk-boundary correspondences of canonical four-band models for topological skyrmion phases and their connection to simpler forms of bulk-boundary correspondence. Given the fundamental roles entanglement signatures and observables play in study of quantum many body systems, observable-enriched entanglement is broadly applicable to myriad problems of quantum mechanics.

We explore the ground states of infinite chains with a large number N of Majorana fermions on each site, interacting via identical on-site Sachdev-Ye-Kitaev (SYK) couplings and inhomogeneous nearest-neighbor hopping terms. Our results unify techniques for solving SYK-like models in the large N limit with a real-space renormalization group method known as strong-disorder renormalization group (SDRG). Decimation steps involve projecting Majorana fermions of strongly coupled sites onto their ground state, inducing effective hopping interactions between neighboring sites. If the two sites are nearest neighbors, these local ground states admit a holographic dual description as eternal traversable wormholes. Otherwise, the wormhole structure fades, inducing non-vanishing correlations between distant degrees of freedom. We apply our methods to two specific instances of these infinite SYK chains with either aperiodically or randomly distributed hopping parameters.

Recently, by considering the crucial role of the interlayer spin exchange interaction of the electrons on dz2 orbitals, the bilayer t-J model has been proposed as a potential minimum model to study the nickelate superconductor La₃Ni₂O₇ under high pressure. To investigate the superconductivity of this model near the quarter filling, we consider the bilayer t-J-J_⊥​ ladder where t is the in-plane electron hopping, J is the in-plain spin interaction, and J_⊥​ is the interlayer spin interaction. Using density matrix renormalization group method, we study the model on the Ly = 2 and 3 bilayer ladders by tuning J_⊥​. We find that by coupling the two layers in both the Luttinger liquid and Luther-Emery liquid states, increased J_⊥​ can induce the dominant s-wave pairing between the two layers. In the special case of Ly = 2 at the quarter filling, the gapped charge mode appears robust with growing J_⊥​, and superconductivity cannot be induced in this case.

I review three characteristic cases of using Raman spectroscopy to study the collective excitations in topological systems. These examples include the chiral spin mode [PRL 119, 136802 (2017)] and chiral excitons [PNAS, 116, 4006 (2019)] in Bi₂Se₃, and the plasmon mode in BiTeI [PRB 109, L041111 (2024)]. The successful application of Raman spectroscopy demonstrates this experimental method as a promising probe of topological systems.

Here we use a unitary mapping, combined with the well-established properties of the attractive Hubbard model to demonstrate rigorously a Hamiltonian with a low temperature pair-density-wave (PDW) phase. We also show that the same mapping, when applied to the widely accepted properties of the repulsive Hubbard model, leads to a Hamiltonian exhibiting triplet d-wave PDW superconductivity and an unusual combination of ferro- and antiferro-magnetic spin correlations. We then demonstrate the persistence of the d-wave PDW in a Hamiltonian derived from the mapping of the extended t-J model in the large-U limit. Furthermore, through strategic manipulation of the nearest-neighbor hopping signs of spin-down electrons, we illustrate the attainability of PDW superconductivity at other momenta. The intertwining of different magnetic and exotic pairing correlations noted here may have connections to experimental observations in spin-triplet candidates like UTe₂.

A promising platform for the quantum control of high-frequency photons are thin-film cavities, with one or several embedded layers of resonant nuclei with suitable Mössbauer transitions in the x-ray range. At grazing incidence, incoming x-rays couple evanescently to the cavity. In turn, the cavity field drives the nuclear transitions.

Here, we investigate theoretically a thin-film cavity design with multiple embedded ⁵⁷Fe layers, such that its inter-layer couplings are mostly restricted to the nearest neighbouring layers by intercalating additional layers with high electron densities. Via the geometrical properties of these domains and control of the evanescent field pattern, we implement alternating coupling strengths between the resonant layers. We show that this leads to an x-ray photonic realization of the non-hermitian Su-Schrieffer-Heeger model and investigate how for certain configurations localized nuclear excitations emerge at the edges of the cavity.